The present invention relates to methods for synthesis of xylenes by catalytic methylation of toluene and benzene, and more particularly relates, in one embodiment, to methods for synthesis of direct, selective synthesis of para-xylene by catalytic methylation of toluene and benzene.
Of the xylene isomers, i.e., ortho-, meta-, and para-xylene, the para-xylene (PX) is of particular value as a large volume chemical intermediate in a number of applications being useful in the manufacture of terephthalates which are intermediates for the manufacture of PET. One source of feedstocks for manufacturing PX is by disproportionation of toluene into xylenes. One of the disadvantages of this process is that large quantities of benzene are also produced. Another source of feedstocks used to obtain PX involves the isomerization of a feedstream that contains non-equilibrium quantities of mixed ortho- and meta-xylene isomers (OX and MX, respectively) and is lean with respect to PX content. A disadvantage of this process is that the separation of the PX from the other isomers is expensive.
Zeolites are known to catalyze the reaction of toluene with other reactants to make xylenes. Some zeolites are silicate-based materials which are comprised of a silica lattice and, optionally, alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term xe2x80x9czeolitesxe2x80x9d includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorus oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms xe2x80x9czeolitexe2x80x9d, xe2x80x9czeolitesxe2x80x9d and xe2x80x9czeolite materialsxe2x80x9d, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, borosilicates, ferrosilicates, and the like.
The term xe2x80x9czeolite, xe2x80x9czeolitesxe2x80x9d, and xe2x80x9czeolite materialsxe2x80x9d as used herein, besides encompassing the materials discussed above, shall also include aluminophosphate-based materials. Aluminophosphate zeolites are made of alternating AlO4 and PO4 tetrahedra. Aluminophosphate-based materials have lower acidity compared to aluminosilicates. The lower acidity eliminates many side reactions, raises reactants"" utilization, and extends catalyst life. Aluminophosphate-based zeolites are often abbreviated as ALPO. Substitution of silicon for P and/or a Pxe2x80x94Al pair produces silicoaluminophosphate zeolites, abbreviated as SAPO.
Processes have been proposed for the production of xylenes by the methylation of toluene using a zeolite catalyst. For instance, U.S. Pat. No. 3,965,207 involves the methylation of toluene with methanol using a zeolite catalyst such as a ZSM-5. U.S. Pat. No. 4,670,616 involves the production of xylenes by the methylation of toluene with methanol using a borosilicate zeolite which is bound by a binder such as alumina, silica, or alumina-silica. One of the disadvantages of such processes is that catalysts deactivate rapidly due to build up of coke and heavy by-products. Another disadvantage is that methanol selectivity to para-xylene, the desirable product, has been low, in the range of 50 to 60%. The balance is wasted on the production of coke and other undesirable compounds.
It has been further demonstrated that alkylaromatic compounds can be synthesized by reacting an aromatic compound such as toluene with a mixture of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2) (synthesis gas) at alkylation conditions in the presence of a catalyst system, which comprises (1) a composite of oxides of zinc, copper, chromium, and/or cadmium; and (2) an aluminosilicate material, either crystalline or amorphous, such as zeolites or clays; as disclosed in U.S. Pat. Nos. 4,487,984 and 4,665,238. Such catalyst systems, however, are not capable of producing greater than equilibrium concentrations of para-xylene (PX) in the xylene-fraction product. Typically, the xylene-fraction product contains a mixture of xylene isomers at or near the equilibrium concentration, i.e., 24% PX, 54% MX, and 22% OX. The lack of para-xylene selectivity in alkylation of toluene with syngas can be caused by (1) the acidic sites on the surface outside the zeolite channels, and/or (2) the channel structure not being able to differentiate para-xylene from its isomers. It would be desirable for the toluene alkylation to be more para-alkyl selective due to the much higher value of PX compared to that of MX and OX. Furthermore, such processes suffer from catalyst deactivation as well. In addition, the prior art disclosed neither syngas alkylation to alkyl aromatic compounds nor syngas selective alkylation to high purity PX using alumino-phosphate-based materials.
It has been recognized that certain zeolites can be modified to enhance their molecular-sieving or shape-selective capability. Such modification treatments are usually called xe2x80x9czeolite selectivation.xe2x80x9d Selectivated zeolites can more accurately differentiate molecules on the basis of molecular dimension or steric characteristics than the unselectivated precursors. For example, silanized ZSM-5 zeolites adsorbed PX much more preferentially over MX than untreated ZSM-5. It is believed that the deposition of silicon oxide onto zeolite surfaces from the silanization treatment has (1) passivated the active sites on the external surface of zeolite crystals, and (2) narrowed zeolitic pores to facilitate the passage of the smaller PX molecules and prevent the bigger MX and OX molecules from entering or exiting from the pores. In this application, the term xe2x80x9cpara-alkyl selectivationxe2x80x9d refers to modifying a catalyst or catalytic reaction system so that it preferentially forms more PX than the expected equilibrium proportions relative to the other isomers.
Zeolite selectivation can be accomplished using many techniques. Reports of using compounds of silicon, phosphorous, boron, antimony, coke, magnesium, etc. for selectivation have been documented. Unfortunately many, if not most of the zeolites used in the prior art have undesirably short active lifetimes before they deactivate and have to be reactivated or replaced.
There remains a need for still further improved processes for catalytic PX synthesis which minimizes or avoids the disadvantages of prior systems, which include low PX selectivity, low methanol selectivity, rapid catalyst deactivation, and the like.
Accordingly, it is an object of the present invention to provide a method in which PX of high product concentration is synthesized via alkylation of toluene and/or benzene with a mixture including H2, and CO and/or CO2 and/or methanol in a catalytic reaction system.
It is another object of the present invention to provide a method for producing PX in greater than equilibrium product concentration, e.g. greater than 30%, in the xylene product fraction as compared to prior, equilibrium concentrations of about 24%.
Still another object of the invention is to provide a method for the direct, selective production of PX from toluene and/or benzene which has a high aromatic conversion, e.g. at least above 5%, preferably above 15%, most preferably as high as possible.
In carrying out these and other objects of the invention, there is provided, in one form, a method for forming para-xylene (PX) involving reacting a feed containing an aromatic compound of toluene, benzene and mixtures thereof with a methylating agent formed from H2, and CO and/or CO2 and/or methanol and mixtures thereof, in the presence of a catalytic reaction system which converts at least 5% of the aromatic compound to a mixture of xylenes, where PX comprises at least 30% of the mixture of xylenes.
It should be stressed that the invention provides a process for increased selectivity to para-xylene. Further, the invention employs aluminophosphate-based catalysts for selective para-xylene synthesis, whether or not the aluminophosphate-based materials are para-alkyl selectivated. Additionally, the invention prolongs catalyst lifetime by reducing, even eliminating catalyst deactivation, as compared with prior PX forming processes. Another advantage of the invention is the simultaneous cracking of paraffins and olefins present while processing unextracted toluene, or unextracted mixtures of toluene and benzene.
The present invention relates to a process to synthesize PX with para-alkyl selective alkylation of toluene and/or benzene with a mixture containing, as predominant components thereof, hydrogen, carbon monoxide, and/or carbon dioxide and/or methanol in the presence of a catalyst system. A method has been discovered by which the product selectivity to para-xylene for an aromatic alkylation process using as alkylating agents mixtures of H2, CO, and/or CO2 and/or methanol is significantly enhanced. The improvement in para-xylene selectivity is achieved by treating the molecular sieve zeolite materials with proper chemical compounds to (1) inhibit the external acidic sites to minimize aromatic alkylation on the non-para positions, and/or the isomerization of the para-alkylated compounds, and/or (2) impose more restrictions on the channel structure to facilitate the formation and transport of para-alkylated aromatic compounds, in one non-limiting explanation of the mechanism of the invention. It must be understood that such treatment may be performed on aluminophosphate catalyst reaction systems of this invention, but that some aluminophosphate catalyst reaction systems do not require this para-alkyl selectivation treatment to be effective at para-alkyl selectivation in the methods of this invention.
The catalytic reaction systems suitable for this invention include (1) a first component of one or more than one of the metals or oxides of the metal elements selected from Groups 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 (new IUPAC notation, e.g. zinc, copper, chromium, cadmium, palladium, ruthenium, manganese, etc.), and (2) a second component of one or more than one of the zeolite or amorphous materials, some of which are selectivated for para-position selectivity. The first and second components may be chemically mixed, physically mixed, and combinations thereof, as will be described.
One type of the zeolite materials would be silicate-based zeolites such as faujasites, mordenites, pentasils, etc.
Zeolite materials suitable for this invention include silicate-based zeolites and amorphous compounds. Silicate-based zeolites are made of alternating SiO2 and MOx tetrahedra, where in the formula M is an element selected from the Groups 1 through 16 of the Periodic Table (new IUPAC). These types of zeolites have 8-, 10-, or 12-membered oxygen ring channels. Silicate-based materials are generally acidic. The more preferred zeolites of this invention include 10- and 12-membered ring zeolites, such as ZSM-5, ZSM-11, ZSM-22, ZSM-48, ZSM-57, etc.
One of the disadvantages to the use of many unselectivated silicate-based materials for such PX synthesis systems is the lack of product selectivity (i.e. an undesirably broad product distribution results). The acidity and structure of many silicate-based materials are such that they often promote many undesirable side reactions (e.g. dealkylation, isomerization, multi-alkylation, oligomerization, and condensation) which deactivate the catalysts and lower the product value. It follows that, silicate-based materials are typically not capable of delivering the shape selectivity for increasing the yield for high-value products such as para-xylene (PX).
Some of the silicate-based materials have one-dimensional channel structures, which are capable of generating higher-than-equilibrium PX selectivity. These materials may optionally be para-alkyl selectivated. Other silicate-based materials having two- or three-dimensional channel structures are preferably para-alkyl selectivated or modified to be more selective through the use of certain chemical compounds, as will be described, such as organometallic compounds and compounds of elements selected from Groups 1-16. In one embodiment, the selectivation of the zeolite materials including the silicate-based materials can be accomplished using compounds including, but not necessarily limited to silicon, phosphorus, boron, antimony, magnesium compounds, coke, and the like, and mixtures thereof.
Other silicate-based materials suitable for the second component include zeolite bound zeolites as described in WO 97/45387, incorporated herein by reference. Zeolite bound zeolite catalysts useful in the present invention concern first crystals of an acidic intermediate pore size first zeolite and a binder comprising second crystals of a second zeolite. Unlike zeolites bound with amorphous material such as silica or alumina to enhance the mechanical strength of the zeolite, the zeolite bound zeolite catalyst suitable for use in the present process does not contain significant amounts of non-zeolitic binders.
The first zeolite used in the zeolite bound zeolite catalyst is an intermediate pore size zeolite. Intermediate pore size zeolites have a pore size from about 5 to about 7 xc3x85 and include, for example, AEL, MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, FER, and TON structure type zeolites.
These zeolites are described in Atlas of Zeolite Structure Types, eds. W. H. Meier and D. H. Olson, Butterworth-Heineman, Third Edition, 1992, which is hereby incorporated by reference. Examples of specific intermediate pore size zeolites include, but are not limited to, ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48, ZSM-50, and ZSM-57. Preferred first zeolites are galliumsilicate zeolites having an MFI structure and aluminosilicate zeolites having an MFR structure.
The second zeolite will usually have an intermediate pore size and have less activity than the first zeolite. Preferably, the second zeolite will be substantially non-acidic and will have the same structure type as the first zeolite. The preferred second zeolites are aluminosilicate zeolites having a silica to alumina mole ratio greater than 100 such as low acidity ZSM-5. If the second zeolite is an aluminosilicate zeolite, the second zeolite will generally have a silica to alumina mole ratio greater than 200:1, e.g., 500:1; 1,000:1, etc., and in some applications will contain no more than trace amounts of alumina. The second zeolite can also be silicalite, i.e., a MFI type substantially free of alumina, or silicalite 2, a MEL type substantially free of alumina. The second zeolite is usually present in the zeolite bound zeolite catalyst in an amount in the range of from about 10% to 60% by weight based on the weight of the first zeolite and, more preferably, from about 20% to about 50% by weight.
The second zeolite crystals preferably have a smaller size than the first zeolite crystals and more preferably will have an average particle size from about 0.1 to about 0.5 microns. The second zeolite crystals, in addition to binding the first zeolite particles and maximizing the performance of the catalyst will preferably intergrow and form an over-growth which coats or partially coats the first zeolite crystals. Preferably, the crystals will be resistant to attrition.
The zeolite bound zeolite catalyst suitable for the process of the present invention is preferably prepared by a three step procedure. The first step involves the synthesis of the first zeolite crystals prior to converting it to the zeolite bound zeolite catalyst. Next, a silica-bound aluminosilicate zeolite can be prepared preferably by mixing a mixture comprising the aluminosilicate crystals, a silica gel or sol, water and optionally an extrusion aid and, optionally, the metal component until a homogeneous composition in the form of an extrudable paste develops. The final step is the conversion of the silica present in the silica-bound catalyst to a second zeolite which serves to bind the first zeolite crystals together.
As noted, aluminophosphate-based materials may be used in conjunction with metal oxides for aromatic alkylation with syngas. Aluminophosphate-based materials usually have lower acidity compared to silicate-based materials. The lower acidity eliminates many side reactions, raises reactants"" utilization, and extends catalyst life. In addition, some of the medium-pore aluminophosphate-based materials have unique channel structures that could generate the desirable shape selectivity.
Further, catalytic reaction systems suitable for this invention include aluminophosphate-based materials and amorphous compounds. Aluminophosphate-based materials are made of alternating AlO4 and PO4 tetrahedra. Members of this family have 8- (e.g. AlPO4-12, -17, -21, -25, -34, -42, etc.), 10- (e.g. AlPO4-11, 41, etc.), or 12- (AlPO4-5, -31, etc.) membered oxygen ring channels. Although AlPO4s are neutral, substitution of Al and/or P by cations with lower charge introduces a negative charge in the framework, which is countered by cations imparting acidity.
By turn, substitution of silicon for P and/or a Pxe2x80x94Al pair turns the neutral binary composition (ie. Al, P) into a series of acidic-ternary-composition (Si, Al, P) based SAPO materials, such as SAPO-5, -11, -14, -17, -18, -20, -31, -34, -41, -46, etc. Acidic ternary compositions can also be created by substituting divalent metal ions for aluminum, generating the MeAPO materials. Me is a metal ion which can be selected from the group consisting of, but not limited to, Mg, Co, Fe, Zn and the like. Acidic materials such as MgAPO (magnesium substituted), CoAPO (cobalt substituted), FeAPO (iron substituted), MnAPO (manganese substituted), ZnAPO (zinc substituted) etc. belong to this category. Substitution can also create acidic quaternary-composition based materials such as the MeAPSO series, including FeAPSO (Fe, Al, P, and Si), MgAPSO (Mg, Al, P, Si), MnAPSO, CoAPSO, ZnAPSO (Zn, Al, P, Si), etc. Other substituted aluminophosphate-based materials include EIAPO and EIAPSO (where EI=B, As, Be, Ga, Ge, Li, Ti, etc.) As mentioned above, these materials have the appropriate acidic strength for syngas/aromatic alkylation. The more preferred aluminophosphate-based materials of this invention include 10- and 12-membered ring materials (SAPO-11, -31, -41; MeAPO-11, -31, -41; MeAPSO-11, -31, 41; EIAPO-11, -31, -41; EIAPSO-11, -31, -41, etc.) which have significant shape selectivity due to their narrow channel structure.
It has been discovered that it may not be necessary for the aluminophosphate-based materials to be processed in a para-alkyl selectivation, step for good selectivity prior to producing para-xylene. Optionally, however, these aluminophosphates may be para-alkyl selectivated or modified to be more selective through the use of certain chemical compounds, as will be described, such as organometallic compounds and compounds of elements selected from Groups 1-16. In one embodiment, the para-alkyl selectivation of the zeolite materials including the aluminophosphate-based materials can be accomplished using compounds including, but not necessarily limited to silicon, phosphorus, boron, antimony, magnesium compounds, coke, and the like, and mixtures thereof.
The composition of the proposed catalytic reaction systems may be from 5 wt. % metals or metal oxides first component/95 wt. % silicate-based material or aluminophosphate-based material second component, to 95 wt. % metals or metal oxides first component/5 wt. % silicate-based material or aluminophosphate-based material second component. The preparation of the catalytic reaction systems can be accomplished with several techniques known to those skilled in the art. Some examples are given below.
Para-alkyl selectivation involves the treatment of the above mentioned catalytic reaction system materials with proper chemical compounds. Some para-alkyl selectivation treatments are known, e.g. using silicon compounds. Other compounds that may be used include, but are not limited to compounds of phosphorus, boron, antimony, magnesium, and the like, and coke, and the like. Para-alkyl selectivation treatments of the materials of the above catalytic reaction systems, such as by using the metals/metal oxides first component to selectivate, can be carried out either prior to the selective formation of PX (ex situ) or during the PX formation (in situ). In the in situ embodiment, the selectivating agents are added with the feed to the reactor containing a catalytic reaction system.
In more detail, in one non-limiting embodiment, the technique for selectivating the materials useful in the method of this invention is based on the consideration that by depositing on a silicate-based material or aluminophosphate-based material one or more than one of the organometallic compounds which are too bulky to enter the channels (or other para-alkyl selectivating agents), one should be able to modify only the external surface and regions around channel mouth. The fact that the para-alkyl selectivation agent does not enter the channels preserves the active sites inside the channels. Since the channel active sites account for the majority of the total active sites, their remaining active prevents any significant loss of reactivity or conversion.
It will be understood that the para-alkyl selectivation techniques of this invention may be practiced before or after the silicate-based materials or aluminophosphate-based materials are mixed with or combined chemically or physically with metals or metal oxides. That is, in some embodiments, the silicate-based materials and aluminophosphate-based materials may be para-alkyl selectivated before combination with metals or metal oxides. In other embodiments, silicate-based materials and aluminophosphate-based materials may be para-alkyl selectivated after combination with metals or metal oxides. The former process might be termed xe2x80x9cpre-selectivationxe2x80x9d, while the latter process may be termed xe2x80x9cpost-selectivationxe2x80x9d.
One type of the bulky organometallic compounds suitable for para-alkyl selectivating 10-member-ring zeolites, such as the ZSM family (e.g. ZSM-5, -11, -22, -48, etc.), mordenite, etc., is the salts of large organic anions and metallic cations. The organic anions can be selected from molecules containing carboxylic and/or phenolic functional groups, including but not limited to phthalate, ethylenediaminetetraacetic acid (EDTA), vitamin B-5, trihydroxy benzoic acid, pyrogallate, salicylate, sulfosalicylate, citrate, naphthalene dicarboxylate, anthradiolate, camphorate, and others. The metallic cations can be selected from the element(s) of Groups 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, and 16 (new IUPAC notation). Other compounds for para-alkyl selectivating the silicate-based and aluminophosphate-based materials include, but are not necessarily limited to silicon, phosphorus, boron, antimony, magnesium compounds, coke, and the like, and mixtures thereof.
Para-alkyl selectivation of silicate-based materials and aluminophosphate-based materials with the above mentioned organometallic salts can be accomplished by various means. For example, one can use impregnation of a solution of an organometallic salt onto a silicate-based material or aluminophosphate-based materials. Either water or any suitable organic solvent can be used. Addition of non-metallic salts and/or adjustments of pH to facilitate the treatment are optional. Heat will be provided to drive off the solvent leaving behind a material coated homogeneously with the organometallic salt. Drying and calcination of the coated zeolite or aluminophosphate-based materials at appropriate temperatures will turn the salt into metal oxide. Alternatively, one can use a dry-mix technique, which involves mixing directly a zeolite in the form of powder or particles with a organometallic salt also in the form of powder or particles without the use of any solvent. The mixture will then be subjected to heat treatment, which facilitates the dispersion of the salt over the material and eventually turn the salt into metal oxide.
Known techniques for ex situ and in situ catalytic reaction system modification can be incorporated into producing the para-alkyl selectivated catalytic reaction systems of the present invention in accordance herewith, such as those seen in U.S. Pat. Nos. 5,476,823 and 5,675,047, incorporated herein by reference.
In one embodiment, the same metals and/or metal oxide components used in the catalytic reaction system can be used alone or together to selectivate the silicate-based material or aluminophosphate-based material components.
The catalytic reaction systems can be prepared by adding solutions of metal salts either in series to or as a mixture with the fine powder or particles such as extrudates, spheres, etc. of the unselectivated silicate-based materials having one dimensional channel structures, or optionally ex situ para-alkyl selectivated silicate-based materials, unselectivated aluminophosphate-based materials, or optionally ex situ para-alkyl selectivated aluminophosphate-based materials, until incipient wetness is reached. The solvent (water or other solvents) can be evacuated under heat or vacuum using a typical equipment such as a rotary evaporator. The final product is dried, calcined, and pelletized, if necessary.
Alternatively, solutions of metal salts and the ex situ para-alkyl selectivated fine powder or particles such as extrudates, spheres, etc. of the silicate-based and aluminophosphate-based materials are thoroughly mixed. A dilute basic solution (e.g. ammonia, sodium carbonate, potassium hydroxide, etc.) is used to adjust the pH value of the mixture to facilitate the precipitation of metal hydroxides and zeolites. The precipitate is filtered and washed thoroughly with water. The final product is dried, calcined, and pelletized, if necessary.
The catalytic reaction systems can also be prepared using physical mixing. Finely divided powders of metal(s) or metal oxide(s), or powders of metal(s) or metal oxide(s) supported on any inert materials, are mixed thoroughly with finely divided powder of the ex situ para-alkyl selectivated silicate-based materials, unselectivated aluminophosphate-based materials, or optionally ex situ para-alkyl selectivated aluminophosphate-based materials in a blending machine or a grinding mortar. The mixture is optionally pelletized before use.
If the catalytic reaction system is mixed with a binder, such as silica gel or sol or the like, an extrudable paste may be formed. The resulting paste can be molded, e.g. extruded, and cut into small strands which can then be dried and calcined.
The catalytic reaction system can also be prepared by mixing physically the particles of silicate-based material or aluminophosphate-based materials components and the particles of the metal and/or metal oxide first components. The same metals and/or metal oxides can also be used alone or together to selectivate catalytic reaction systems and thus simultaneously catalyze syngas reactions.
The catalytic reaction system can also be formed by packing the first and the second components in a stacked-bed manner with some of the first component in front of the physical or chemical mixture of the first and second components.
Prior to exposing the catalytic reaction systems to the feed components of toluene and/or benzene, H2, CO, and/or CO2 and/or methanol the catalytic reaction systems can optionally be activated under a reducing environment (e.g. 1-80% H2 in N2) at 150-500xc2x0 C., and 1-200 atm (1.01xc3x97105-2.03xc3x97107 Pa) for 2-48 hours.
The average crystal size of the crystals in the silicate-based material or aluminophosphate-based materials is preferably from above 0.1 micron to about 100 microns, more preferably from about 1 micron to about 100 microns.
Procedures to determine crystal size are known to persons skilled in the art. For instance, crystal size may be determined directly by taking a suitable scanning electron microscope (SEM) picture of a representative sample of the crystals.
The methylation process can be carried out as a batch type, semi-continuous or continuous operation utilizing a fixed, moving bed, or CSTR catalytic reaction system, with or without recycle. Multiple injection of the methylating agent may be employed. The methylating agent includes CO, CO2 and H2 and/or CH3OH and derivatives thereof. The methylating agent reacts with benzene to form toluene. Toluene reacts with the methylating agent to form a xylene, preferably PX in this invention. In one preferred embodiment of the invention, methanol as the methylating agent is not separately added but is formed in situ.
Toluene and/or benzene and the methylating agent(s) are usually premixed and fed together into the reaction vessel to maintain the desired ratio between them with no local concentration of either reactant to disrupt reaction kinetics. Individual feeds can be employed, however, if care is taken to insure good mixing of the reactant vapors in the reaction vessel. Optionally, instantaneous concentration of methylating agent can be kept low by staged additions thereof. By staged additions, the ratios of toluene and/or benzene to methylating agent concentrations can be maintained at optimum levels to give good aromatic compound conversions and better catalytic reaction system stability. Hydrogen gas can also serve as an anticoking agent and diluent.
The method of this invention, particularly when using para-alkyl selectivated catalytic reaction systems, stabilizes catalytic reaction system performance and increases catalytic reaction system life. That is, catalytic reaction system deactivation is slowed and even prevented. With properly para-alkyl selectivated catalytic reaction systems, it is expected that the catalytic reaction system may not have to be regenerated at all. This is in part due to the silicate-based materials and aluminophosphate-based materials being para-alkyl selectivated. With production selective to PX, less by-products, such as heavy aromatics, are formed which would deactivate the catalytic reaction systems. This characteristic is not shown or taught by the prior art.
Further, in one non-limiting embodiment of the invention, there is a belief that the catalytic reaction systems of this invention have the capability of preventing or reducing the side reactions of the methylating agents with themselves, and in particular that the para-alkyl selectivated catalytic reaction systems function to catalyze more than one reaction, that is, that a syngas reaction is catalyzed to form methylating agents which react with benzene and/or toluene to produce PX. However, because the methylating agent is produced on a local, molecular scale, its concentrations are very low (as contrasted with feeding a methylating agent as a co-reactant). It has been demonstrated that feeding a blend of methylating agent and toluene to make PX increases coke build-up and hence catalytic reaction system deactivation.
In one non-limiting embodiment of the invention, the catalyst activity decrease is less than 0.5% toluene and/or benzene conversion pet day, preferably less than 0.1%.
In carrying out the process, the feed mixtures can be co-fed into a reactor containing one of the above mentioned catalytic reaction systems. The catalytic reaction system and reactants can be heated to reaction temperatures separately or together. Reaction can be carried out at a temperature from about 100-700xc2x0 C., preferably from about 200-600xc2x0 C.; at a pressure from about 1-300 atm (1.01xc3x97105-3.04xc3x97107 Pa), preferably from about 1-200 atm (1.01xc3x97105-2.03xc3x97107 Pa); and at a flow rate for about 0.01-100 hxe2x88x921 LHSV, preferably from about 1-50 hxe2x88x921 LHSV on a liquid feed basis. The composition of the feed, i.e. the mole ratio of H2/CO(and/or CO2)/aromatic can be from of about 0.01-10/0.01-10/0.01-10, preferably from about 0.1-10/0.1-10/0.1-10.
As noted, typical methylating agents include or are formed from, but are not necessarily limited to hydrogen together with carbon monoxide and/or carbon dioxide, and/or methanol, but also dimethylether, methylchloride, methylbromide, and dimethylsulfide.
It is conceivable that in the scenarios described above, the toluene can be pure, or in a mixture with benzene. The benzene may alkylate to toluene, and/or ultimately to PX, with or without recycle. The presence of benzene may also enhance heat and/or selectivity control.
The method of this invention is expected to tolerate many different kinds of feed. Unextracted toluene, which is a mixture of toluene and similar boiling range olefins and paraffins, is preferred in one embodiment. For example, premium extracted toluene, essentially pure toluene, and extracted aromatics, essentially a relatively pure mixture of toluene and benzene, may also be used. Unextracted toluene and benzene which contains toluene, benzene, and olefins and paraffins that boil in a similar range to that of toluene or benzene, may also be employed. When unextracted feedstocks are used, it is important to crack the paraffins and olefins into lighter products that can be easily distilled. For example, the feed may contain one or more paraffins and/or olefins having at least 4 carbon atoms; the catalytic reaction systems have the dual function to crack the paraffins and/or olefins and methylate benzene or toluene to selectively produce PX.
Indeed, some of the catalytic reaction systems of this invention may be multifunctional in some embodiments, catalyzing a reaction or reactions of CO, H2, and/or CO2 and/or methanol to produce a methylating agent, catalyzing the selective methylation of toluene and/or benzene to produce PX, and catalyzing the cracking of paraffins and olefins into relatively lighter products.
The method of this invention is capable of producing mixtures of xylenes where PX comprises at least 30 wt. % of the mixture, preferably at least 36 wt. %, and most preferably at least 48 wt. %. The method of this invention is also capable of converting at least 5 wt. % of the aromatic compound to a mixture of xylenes, preferably greater than 15 wt. %.
para-Xylene may be recovered from the process stream, for example by crystallization, for use in products such as terephthalic acid, dimethyl terephthalate acid, polyethylene terephthalate polymer, and the like, which in turn can be used to make synthetic fibers. There are three commercial techniques to recover PX, fractionation, adsorption (PAREX zeolite), and crystallization. In a preferred embodiment of the invention, combinations of these recovery techniques may used to lower capital costs. In another preferred embodiment of the invention, crystallization is used, particularly single-stage crystallization. Single-stage crystallization simply means that only one crystallization step is used on the product from the inventive process, which would be a simple and relatively inexpensive procedure. Because of the high quality product produced by the inventive process, it is expected that the PX proportion in the product from the inventive process may be 80% or more, while after one crystallization step, the proportion may be 99% or higher.
The following examples will serve to illustrate the processes and merits of the present invention. It is to be understood that these examples are merely illustrative in nature and that the present process is not necessarily limited thereto.